The invention relates to protection techniques for electric circuits, and more particularly to the detection of arcs which are too small to trip a circuit breaker or the like.
The invention arose during development efforts in electric circuit protection techniques, including residential, commercial and industrial applications having overcurrent protection and/or ground fault interrupter (GFI) systems for preventing property damage and/or personal injury. Overcurrent protection systems typically include a load center or panelboard receiving electrical power from the utility company transformer, and routing the electrical power through a main circuit breaker or fuse and then through branch circuit breakers or fuses to designated branch circuits, each supplying current to one or more electrical loads. The circuit breakers or fuses are designed to interrupt the electrical current if it is excessive or outside the design limits of the conductors and loads or demonstrates a current imbalance, to reduce the risk of injury and damage, including fires. Circuit breakers are more commonly preferred because they are reusable simply by resetting same.
A circuit breaker has a thermal/magnetic trip characteristic. The thermal characteristic is operative in response to overload current of extended duration which heats a bimetal member, causing movement of the latter, which in turn releases a latch to trip open a set of contacts. For example, the thermal characteristic would respond to 30 amps being drawn in a 15 amp circuit. The magnetic characteristic is operative in response to a sudden high magnitude current overload condition, and uses the magnetic field generated in a magnetic core to attract an armature, which movement releases the latch to open the contacts. As an example, the magnetic type actuation occurs in response to a short circuit wherein the hot line conductor becomes directly connected with ground or neutral, bypassing the load.
A further type of circuit protection is provided by a ground fault interrupter, which trips the breaker to an open circuit condition in response to an imbalance of currents between the line and neutral conductors of a given circuit. This is particularly desirable in the event that a person is in the path to ground.
The present invention addresses a further type of electric circuit protection and monitoring, namely the detection of arcing in the electric circuit. The arcs to be monitored include not only a discharge of electricity through a gas or across an insulating medium, but also high impedance faults or other intended or unintended circuit paths which do not have sufficient energy or current flow to trip a breaker, but nevertheless can generate damaging heat or other undesirable effects. An arc may occur in series or parallel with the load, and may have numerous causes, for example loose connections, worn or aged wiring or insulation, prior mechanical or electrical stressing such as caused by crimping or twisting, overuse, prior lightning strikes, etc. The current in such arcs may be below the thermal and magnetic trip settings of the breaker or a fuse, and/or may not present an imbalance of currents between the line and neutral conductors, and hence such arcs can go undetected. A particular hazard due to arcing is hot spots along electrical wiring in a residence or the like, which is a fire risk. This wiring can include wall conductors, junction boxes, receptacles, switches, plugs, cords such as extension cords, and the load devices themselves.
The present invention provides effective arc detection. The invention is particularly well suited to discriminating between potentially damaging arcs and either non-arcing events or momentary short duration switching events, which is desirable in order to prevent nuisance tripping. The invention does not trip in response to all arcs, nor all seemingly arc-like events, but rather relies upon statistical probabilities to provide an arc indicative signal in response to an event which has a strong likelihood of being an arc. To attempt to respond to all arcs would involve an unacceptably high occurrence of nuisance tripping, which is undesirable because it may shut down machinery or safety equipment which should remain running, except for those events actually having a high statistical probability of being an arc.